Discharge lamp with cathode shields



Feb. 11, 1964 R. 5. Fox 3,121,184

DISCHARGE LAMP WITH CATHODE SHIELDS Filed Dec. 30, 1960 INVENTOR.

Faber Z 5. Fx

BYW

United States Patent 3,121,184 DISCHARGE LAMP WITH CATHQDE SEMLDS RobertS. Fox, Cleveland Heights, Ohio, assignor to General Electric Company, acorporation of New York Filed Dec. 30, 1960, Ser. No. 79,650 3 Claims.(Cl. 313-207) This invention relates to low-pressure electric dischargelamps such as fluorescent lamps comprising a pair of thermionicelectrodes sealed into opposite ends of an elongated tube containingmercury vapor and an inert gas as the ionizable medium. The invention ismore particularly concerned with high efliciency fluorescent lampsutilizing shields or anode plates about the thermionic electrodes whichare correlated in design to the thermal inertia and operatingtemperature of the electrodes so as to achieve reduction of the anodevoltage drop and prevention of end darkening of the lamp without anyacceleration of damage to the electrodes.

In copending application Serial No. 88,228 filed February 9, 1961, nowPatent No. 3,069,581 by Eugene Lemmers, entitled Low Pressure DischargeLamp and assigned to the same assignee as the present invention, whichapplication is a continuation of an earlier application Serial No.812,235 filed May 11, 1959, now abandoned, there is described andclaimed a high efliciency low pressure discharge lamp such as afluorescent lamp having shields or anode plates about the thermionicelectrodes. Prior to the invention of said Lemmers application, shieldshad been used about cathodes to trap sputtered or vaporized electrodematerial and prevent it from darkening the bulb wall. However theLemmers invention determined that shields as used in the past actuallyshortened the life of the cathode, and that such shields While hidingthe damage actually accelerated it. Lemmers taught that the reduction incathode life entailed by the presence of a shield around the cathode canbe avoided by reducing the size or thermal mass of the cathode so as toachieve the optimum cathode operating temperature despite the presenceof the shields and their tendency to rob ions from the sheath and tosubtract current from the cathode. According to the Lemmers invention,shields are used in combination with a smaller filamentary cathode whichis designed to operate in the usual manner without shields at 50 to62.5% of the normal discharge current. A cathode so designed may then beused with shields in a lamp operating at normal current, that is at 100%discharge current, and normal cathode life will be experienced. Ineffect, according to the Lemmers invention, the cathode is designed asregards resistance and thermal mass including its disposition to loseheat by radiation and otherwise, to achieve an emission spot temperaturein the range of 1050 to 1200 C. while providing 100% of the electronemission on the cathode half-cycle and only 0 to 25 of the electroncollection on the anode half-cycle. Since each half-cycle provides 50%of the normal discharge current, it follows that the filamentarycathodes are designed for normal operation without shields at 50 to62.5% of the normal discharge current. The balance of electroncollection on the anode halfcycle is made up for the most part by theshields with possibly a minor fraction by the inleads which support thecathode and the shield. The shields or anode plates are spaced farenough away from the cathode, a minimum of approximately 3 millimeters,that ion trapping has a negligible effect on cathode fall.

The object of this invention is to provide a further improvement inefficiency in a low-pressure electric discharge lamp utilizing ashielded cathode structure ac cording to the principles of theaforementioned copending Lemmers application.

In the preferred embodiment described and illustrated 3,l2l,l84 PatentedFeb. 11, 1964 in the aforementioned copending Lemmers application, theshield comprises a pair of anode plates disposed on either side of thefilament and transversely to the longitudinal axis of the lamp andgenerally centered about the filament. The resulting configurationserves to more or less box in or encircle the electrode and a transverseplane through the filament would pass medially through the cathodeshields. According to the invention, I have now discovered that afurther improvement in efiiciency may be achieved by displacing theshields rearwardly along the longitudinal axis of the lamp so that thefront or leading edges of the shield plates are approximately even witha plane through the filament transverse to the longitudinal axis of thelamp. By so doing, a consistent improvement in efliciency ofapproximately 1% over the shield disposition centered on the filamenthas been observed in actual production. Furthermore whereas it mighthave been expected that by moving the shield plates rearwardly, theeffectiveness of trapping of sputtered material would be drasticallyreduced, possibly by as much as 50%, in fact very little if any increasein bulb wall darkening has been observed.

For further features and advantages of the invention and for a moredetailed description of a preferred embodiment thereof, attention is nowdirected to the following description and accompanying drawing. Thefeatures of the invention believed to be novel will be more particularlypointed out in the appended claims.

In the drawing:

FIG. 1 is a partially cutaway perspective view of a fluorescent lamprepresentative of the invention.

FIG. 2 illustrates the mount and electrode structure of the lamp in sideelevation.

FIG. 3 illustrates the same mount and electrode structure in end view.

FIG. 4 illustrates pictorial-1y the same mount as the lamp closure withattached base.

Referring to FIG. 1, the low pressure discharge lamp 1 embodying theinvention may correspond, in regards to its size and generalconfiguration, to the ordinary 40- watt rapid start fluorescent lamp of48" nominal length and 1 /2" diameter. The lamp comprises an elongatedcylindrical envelope 2 having shouldered ends to which are secured bases3 each provided with a pair of insulated contact terminals or pins 4, 5.As shown at the end of the lamp where a fragment of the envelope wallhas been broken out and in greater detail in FIGS. 2 to 4, electrodemount 6 comprises a relatively short stem tube 7 having its flared outerend 8 sealed peripherally into the shouldered tube end and having apress 9 at its inner end through which are sealed current inlead wires10, 11. The inward projections of the inlead wires into the envelopesupport the filamentary cathode 12 transversely to the longitudinal axisof the discharge channel. The outward projections of the inleads areconnected to the terminal pins 4, 5. In addition the transverseextensions of the inleads at the inner ends support the cathode shields13, 14. The other end of the lamp is provided with a similar cathode andat least one of the stem flares is provided with an exhaust tube 15which is sealed or tipped off in the usual fashion. The illustratedcathode consists of a multiple coiled tungsten filament coated withactivating material such as alkaline earth oxides including bariumoxide. The illustrated filament is double coiled; other forms offilamentary cathode may be used such as a triple coiled filament or adouble coiled filament with overwind. In any event, the cathode isproportioned in size to achieve an emission spot temperature in therange of 1050 to 1200 C. while providing of the electron emission on thecathode half cycle and 0 to 25% of the electron collection on the anodehalf cycle, as previously stated.

The lamp contains a quantity of mercury indicated by droplet 16exceeding in amount the quantity vaporized during operation of the lamp.In addition a filling of an inert gas, for instance argon with a minorproportion of neon, is provided at a total pressure of a few millimetersof mercury. At relatively low loadings in the range of to 15 watts perfoot, an increase in efliciency may be achieved by replacing up toapproximately 50% of the argon filling gas by neon in the pressure rangefrom 1.5 to 3 millimeters of mercury. At a pressure of approximately 2millimeters, the preferred proportions of neon is in the range from 25to 40%, the latter being preferred where the emphasis is on maximumoutput, and the former where the emphasis is on maximum life. For theinstant 40-watt lamp having a nominal loading of watts per foot,'thepreferred filling gas mixture consists of 70% argon and 30% neon at atotal pressure of approx mately 2.5 millimeters of mercury. A phosphorcoating indicated at 17 on the inside of the envelope wall converts theultraviolet radiation of the discharge through the mercury vapor intovisible light.

As illustrated, the shield assembly comprises a pair of L-shaped metalplates 13, 14. Each plate comprises a main portion 13 which is disposedgenerally parallel to the filament 12 and transversely to thelongitudinal axis of the lamp, and an auxiliary portion 19 which extendsfrom the main portion at an angle slightly greater than 90. The shieldplates are located about at the boundary of the cathode glow and in anycase at a distance greater than approximately 3 millimeters from theemitting surface of the cathode. In a preferred construction suitablefor a 40-watt fluorescent lamp, a distance d is 5 to 7 millimeters andwith this spacing, the ion trapping effect of the shield has negligibleeffect on the cathode fall. The flat surfaces of the plates extendparallel to the longitudinal axis of the discharge channel.

According to the present invention, the shield is lo eated so that thefront or leading edge of the shield plates 13, 14 are locatedapproximately in a transverse plane passing through the filament. Inother words, the shield plates, instead of being medially centered on atransverse plane through the filament, are moved back along the axis ofthe lamp a distance approximately half the width W by comparison withthe preferred disposition shown in the aforementioned copending Lemmersapplication. By so doing, I have found through large scale productiontests a consistent improvement of approximately 1% in efiiciency. Rathersurprisingly, despite the fact that the filament is no longer more orless centrally located in the box-like enclosure formed by the shieldplates, very little increase in bulb wall darkening has been observed.It appears that vaporized or sputtered material from the filamentcollects on the shield plate in preference to the bulb wall and thatfrom the point of view of prevention of envelope darkening, the exactlocation of the shield plates is not too critical. On the other hand,from the point of view of achieving maximum efficiency or maximum lightoutput, the present disposition of the shield is important and rathercritical. By moving the shield plate rearwardly to place their leadingedges in the transverse plane of the filament, the arc length on theanode halfcycle is lengthened so that light output is increased. On theother hand, on the cathode half-cycle, less of the radiation produced bythe negative glow about the cathode is trapped by the shield plates sothat again the light output is increased. Moving the cathode shieldplates further back along the axis of the lamp, that is further to therear of the transverse plane through the filament does not provide anyfurther additional benefit, and may result in undesirable transfer ofthe anode current from the shield plates back to the filament.

As an example of cathode shields in accordance with the inventionsuitable for a 40-watt fluorescent lamp, the shield plates 13, 14 mayconsist of thin nickel strips each having a width W of approximately 9mm. and a total length L of approximately 21 mm. The short leg 19 ofeach strip welded to the inlead is approximately 6 mm. long and the longleg 13 approximately 15 mm. long. The distance D between the long legsof the strips is approximately 14 and the long legs are preferablypositioned parallel to the filament as illustrated. The distance d fromthe emitting surface of the filament to the long legs of the shieldstrip is approximately 6 mm, a distance substantially greater than theminimum of 3 mm. required to reduce ion trapping to the point where ithas negligible effect on cathode fall. The total inside surface of theshield members l3, l4 surrounding the cathode is approximately 3.8 cm.The cathodes in lamps of this configuration carry from 8 to 15% of thedischarge current on the anode half-cycle, the exact figure dependingupon the usual lamp variables such as quality of exhaust, inert gas fillpressure, and quality of cathode activation. Thus the cathodes carryfrom 54 to 57.5% of the total lamp current. The improvement inefliciency over similar prior lamps differing only by having the shieldsmedially disposed on the transverse plane through the filaments is about1% or a gain in output of about 30 lumens in a 40 watt lamp producing3000 lumens.

The shield plates are preferably made of a metal which forms a good basemember for activation by barium or barium oxide which is vaporized fromthe filament during operation and which deposits on the shield plate.Suitable metals are tungsten, molybdenum, tantalum and nickel: nickel ispreferred because it is cheapest. If desired, the shields may be made ofperforated strip or of screening in order to reduce trapping of 25 37 A.radiation originating from the cathode glow, and the term plate as usedin the clahns to follow is intended to cover such variation. Also,instead of a pair of L-shaped plates at each cathode, a pair of C-shapedplates may be used forming a generally circular ring-like shield aboutthe filament, the front or leading edges of the plates or segments beinglocated approximately in a plane transverse to the longitudinal of thedischarge channel and passing through the filament.

While a specific embodiment of the invention has been illustrated anddescribed in detail, the same is to be taken as illustratve and not inorder to limit the invention thereto. The invention is of course equallyapplicable to other styles of discharge lamps having an elongateddischarge channel which may not be straight and linear, such asconfigurated re-entrant cross-section lamps, curvilinear circline lamps,and labyrinthine panel type lamps. The scope of the invention is to bedetermined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of filamentaryactivated cathodes supported on inlead wires sealed into opposite endsand mounted transversely to the longitudinal axis of said dischargechannel, said envelope containing an ionizable medium comprising mercuryvapor and an inert starting gas, said cathodes being proportioned inresistance and thermal mass to achieve an electron emission spottemperature in the range of 1050 to 1200" C. with emission of dischargecurrent on the cathode half-cycle and 0 to 25% collection on the anodehalf-cycle, and conductive shields fastened to said inleads encompassingthe cathode glow region of said cathode and collecting substantially thebalance of discharge current, said shields being arranged with surfacesextending substantially parallel to the longitudinal axis of the channelthereat at a distance greater than 3 mm. from the center of thefilaments and with the leading edge of the shields approximately evenwith a plane through the filament transverse to said longitudinal axis.

2. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of cathodes sealedinto opposite ends and containing an ionizable medium comprising mercuryvapor and an inert starting gas, each cathode comprising a multiplecoiled tungsten filament coated with alkaline earth oxide activatingmaterial, supported at opposite ends by inlead wires sealed through saidenvelope, and mounted transversely to the longitudinal axis of thedischarge channel thereat, said cathodes being proportioned inresistance and thermal mass to achieve in operation an electron emissionspot temperature in the range of 1050 to.l200 C. with 100% emission ofdischarge current on the cathode half-cycle and 0 to 25% collection onthe anode half-cycle, and conductive shields fastened to said inleadsencompassing the cathode glow region of said cathode and collectingsubstantially the balance of discharge current, said shields consistingof plates of metal subject to activation by said activating materials,said plates being arranged with surfaces extending substantiallyparallel to the longitudinal axis of the channel thereat at a distancegreater than 3 mm. from the center of the filaments and with the leadingedges of the shield plates approximately even with a plane through thefilament transverse to said longitudinal axis.

3. A low pressure electric discharge lamp comprising a vitreous envelopedefining an elongated discharge channel having a pair of electrodestructures sealed into opposite ends and containing an ionizable mediumcomprising mercury vapor and an inert starting gas at a low pressure,each electrode structure comprising a multiple coiled tungsten filamentcoated with activating material and supported at opposite ends by inleadwires sealed through said envelope, said filaments being proportioned inresistance and thermal mass to achieve in operation an electron emissionspot temperature in the range of 1050 to 1200 C. with emission ofdischarge current on the cathode half-cycle and 0 to 25% collection onthe anode half-cycle, said filaments being mounted to extendtransversely to the longitudinal axis of the discharge channel thereat,conductive shields fastened to said inleads and encompassing the cathodeglow region of said electrodes and consisting of a pair of generallyL-shaped metal plates subject to activation by deposition or" saidactivating material thereon and of a total area sufiicient to collectsubstantially the balance of discharge current, said shield plates beingdisposed with the long leg of the 1. approximately parallel to thefilament and such that the fiat portions thereof are located in planesparallel to the longitudinal axis of the channel and the leading edgesthereof are located to lie approximately in a plane transverse to thelongitudinal axis of the discharge channel thereat and passing throughthe filament.

References Cited in the file of this patent UNITED STATES PATENTS2,930,919 Wainio Mar. 29, 1960

1. A LOW PRESSURE ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS ENVELOPEDEFINING AN ELONGATED DISCHARGE CHANNEL HAVING A PAIR OF FILAMENTARYACTIVATED CATHODES SUPPORTED ON INLEAD WIRES SEALED INTO OPPOSITE ENDSAND MOUNTED TRANSVERSELY TO THE LONGITUDINAL AXIS OF SAID DISCHARGECHANNEL, SAID ENVELOPE CONTAINING AN IONIZABLE MEDIUM COMPRISING MERCURYVAPOR AND AN INERT STARTING GAS, SAID CATHODES BEING PROPORTIONED INRESISTANCE AND THERMAL MASS TO ACHIEVE AN ELECTRON EMISSION SPOTTEMPERATURE IN THE RANGE OF 1050 TO 1200* C. WITH 100% EMISSION OFDISCHARGE CURRENT ON THE CATHODE HALF-CYCLE AND 0 TO 25% COLLECTION OFTHE ANODE HALF-CYCLE, AND CONDUCTIVE SHIELDS FASTENED TO SAID INLEADSENCOMPASSING THE CATHODE GLOW REGION OF SAID CATHODE AND COLLECTINGSUBSTANTIALLY THE BALANCE OF DISCHARGE CURRENT, SAID SHIELDS BEINGARRANGED WITH SURFACES EXTENDING SUBSTANTIALLY PARALLEL TO THELONGITUDINAL AXIS OF THE CHANNEL THEREAT AT A DISTANCE GREATER THAN 3MM. FROM THE CENTER OF THE FILAMENTS AND WITH THE LEADING EDGE OF THESHIELDS APPROXIMATELY EVEN WITH A PLANE THROUGH THE FILAMENT TRANSVERSETO SAID LONGITUDINAL AXIS.